Long-duration, High Altitude Airships (HAA) powered by solar cells have been proposed for both commercial and military applications. For example, station-keeping lighter-than-air HAAs have been proposed to replace cell towers for wireless telephone and data systems. Military applications include telecommunication applications as well as intelligence gathering and radar platforms. In most such applications, long-duration station-keeping is essential. Thus, the airship must be equipped to provide power to operate payloads and on-board systems while simultaneously making headway into the wind in order to hold a fixed geostationary position. Winds at altitude are generally benign, but during brief periods, especially in the higher latitudes, winds can exceed 90 knots for a week or more.
Solar cells have been used to provide power for the airships, however, solar radiation is only available during the day so solar-powered concepts must store energy for use at night and provide sufficient excess storage to react any wind, day or night, storm or calm. Such storage systems are typically heavy, have short cycle life, are complex and unreliable, and/or are depletable.
Blackbody (BB) radiation from the earth is continuous, day and night, with most of the energy in the wave length range from 8 to 40 microns. BB radiation offers a power resource for the airship because the stratospheric temperature is well below the earth's radiation temperature and so the Stefan-Boltzmann Law allows us to produce electrical power via the photovoltaic effect, but limited to Pwrmax=ε·σ·(Te4−Ts4) where ε is the emissivity, σ the Stefan-Boltzmann constant, Te the earth's temperature and Ts the temperature of the sink. Assuming the sink temperature is the ambient air temperature at the airships' flight altitude (say, 217° Kelvin at 65,000 ft) and the earth's radiation temperature is 280° Kelvin, the maximum available power is approximately 223 watts per square meter. Note that ‘sink temperature’ varies a few degrees depending on weather, latitude, climate, etc, and ‘radiation temperature’ varies between 255° K. and somewhat more than 300° K. in a complex way depending on the terrain or ocean underneath the vehicle, the presence of cloud cover, and other factors. Accordingly, the temperatures given are indicative only.
Power-generating photovoltaic cells are not currently available that can deliver electrical power from such long wavelengths. If such cells could be made, they could potentially supply the high-altitude airship from the earthshine alone, continuously, day and night, without the need for solar cells or electrical energy storage.
In recent years, practitioners have been able shift the wave length of impinging infrared radiation on thermo-photovoltaic cells by means of selective emitters and photon converters with equivalent function. Photon converters (of which selective emitters are a subset) absorb photons over a broad range of energies and emit them over a designer-selected narrow band of energy, conserving energy and entropy so as to comply with the First and Second Laws of Thermodynamics.
Thermo-photovoltaic (TPV) cells convert photons to electrical energy. Generally, a TPV system includes a heat source, a photon converter, and a TPV cell. The photon converter (e.g., ‘selective emitter’), through physical (or radiant) contact with the heat source, gains energy, and then re-emits that energy in the form of photons of a selected frequency. These photons are then transmitted to an adjacent solid-state TPV generator and converted into electrical energy. The efficiency of the generator depends primarily upon the Second Law of Thermodynamics (referred to as the ‘Landsberg Limit’ in current TPV practice) and secondarily upon how well the TPV cell's spectral response and the photon converter's output spectrum are matched. The Landsberg Limit is completely determined by the equivalent temperature differential between the source and the sink:
  η  =      1    -                  4        3            ⁢                        T          s                          T          e                      +                  1        3            ⁢                        T          s          4                          T          e          4                    where Te and Ts are the earthshine radiation temperature and the stratospheric sink temperature respectively. In most applications, then, the Landsberg efficiency is beyond the control of the designer. The match between the converter emission spectrum and the TPV spectral response, then, determines the degree to which the TPV cell can reach the Landsberg Limit.
Generally, TPV cells have a narrow band of response that is higher in frequency than the radiation from earthshine. Photons having too long a wavelength cannot be converted into electrical energy and produce unwanted heat, thereby reducing the operating efficiency of the TPV cell. Photons with too short a wavelength will produce electricity in the TPV cell but carry excess energy that must be dissipated as heat and thus also lowers conversion efficiency. Multi-junction TPV cells (called ‘tandem’ cells) have been built that are essentially several different kinds of TPV cell placed in series so that the frequencies not absorbed by the first cell, are absorbed in one of the subsequent cells. Tandem cells can theoretically achieve the Landsberg Limit assuming an infinite number of cells in tandem each with an optimum sensitivity to progressively longer wave lengths. Three junction TPV cells have been built and achieve slightly more than ⅓ of the Landsberg Limit.
Quantum Dots (QDs) are very small semiconductor structures (of the order of nanometers or somewhat larger in diameter) surrounded by a material of a wider bandgap so that the ensemble can be utilized as a TPV cell. Bandgap is defined as the minimum energy necessary for an electron to transfer from the valence band into the conduction band, where it moves more freely and is available as electric power. QDs confine electrons and holes in three spatial dimensions and to a very small number of energy levels, depending on their size. A QD is larger than an atom but behaves as if it were one, releasing its trapped electron-hole pair to an adjacent conductor when it captures an incident photon. QDs in this sense emulate TPV junctions, holding onto electron-hole pairs, receiving photons and directing them toward raising electron-hole energy, and finally injecting the pair into a conductor; they are surrogate molecules except on a somewhat larger scale, where their design features can be controlled. A TPV device can be made from QD structures, semi-conductor junctions, or a combination of both, to achieve the same performance.